In a continuing effort to understand the basis for the act of recombination by addition observed reproducibly in transformation reactions involving capsular markers in pneumococcus, emphasis will be placed in the next year on experiments aimed at determining whether the results of transformation depend on pre-existing differences in DNA in a preparation or on a decision made at the time of the recombinational act. Methods will involve fractionation of DNA, treatment of DNA with selected nucleases and a search for mutants that might be able to exchange, but not add on, DNA.